One piece theories

First off, I'd like to say that I'm a big one piece fan and I've always wondered ahead of the current chapters and episodes, and think up theories for the future. 

Here's my theory about shanks. Now I think shanks is actually a sort of guide to luffy getting the one piece and a behind the scenes mentor. I think the straw hat originally was not given to shanks because Roger saw something in him but rather given to him to find a future successor and entrusted to shanks. So rather than a candidate for pirate king, shanks was just trusted highly enough by Roger to make a judgement.  In the future I do believe that shanks and luffy will clash but rather than as enemies, it will be more of a mentor thing where they may fight either for the straw hat or something else. If u remember in the romance dawn arc shanks lost his hand and then when white beard asked him what he lost his hand to, he said something like "to the new generation". So even from back then he saw something in luffy that gave him the indication of a future pirate king. 
A other theory I got is that the original owner of laws devil fruit may have granted kaido his immorality but I'm unsure whether he got it through that way or maybe it's just a devil fruit ability he possesses. 

So if anyone got any theories of their own. Please state them cos they are interesting.
No one is interested at all in one piece. Hmm